Supercharged internal combustion engines, and, in particular, fuel-injected engines having exhaust-driven superchargers can produce heavy and objectionable exhaust smoke and other noxious emissions when the engine is accelerated rapidly. This can occur if the operator is able to move the engine's fuel quantity control member, such as a fuel injection pump rack, in the fuel-increasing direction faster than the time it takes for the supercharger to build up enough rotational speed to provide sufficient air for combustion of all the additional fuel being delivered. This results in the temporary expulsion of large quantities of unburned fuel as exhaust smoke.
Moreover, fuel-injected engines equipped with exhaust-driven superchargers can create much smoke under a lugging condition. A lugging condition is encountered when the resistance or load on the engine is increased to the extent that the engine rotational speed is reduced below that which is indicated by the governor throttle setting. In a lugging condition, the engine's speed-sensitive governor attempts to regain the engine rotational speed indicated by the governor throttle setting by automatically advancing the fuel quantity control member to supply more fuel. Again, incomplete combustion of the additional fuel may momentarily occur due to an insufficient amount of air being supplied by the supercharger which is initially slowed down during the lugging condition.
Air-fuel ratio control devices are known for automatically preventing an increase in fuel supply during engine operation when the boost or air intake manifold pressure is too low to provide enough air to support complete combustion of that increased fuel supply. For example, in U.S. Pat. No. 4,149,507 issued to Little, Jr. et al on Apr. 17, 1979, such devices may include an integral servo piston and valve unit which during engine operation is hydraulically placed in a restraining relationship with the fuel quantity control member. In order to facilitate dependable starting of the engine, such an integral servo piston and valve unit is adapted to be inoperative during engine shutdown to restrain the fuel quantity control member so that the unrestrained fuel quantity control member can be moved to an excess or maximum fuel delivery position. The unit remains inoperative until such time as a predetermined air intake manifold pressure is attained in response to which the integral servo piston and valve unit moves to a position which permits metering of engine lubrication oil therethrough and activation of the air-fuel ratio control device.
A number of problems are encountered with the above air-fuel ratio control device. First, the integral servo piston and valve unit makes the air-fuel ratio control device complex and expensive to manufacture. Second, faulty or unstable operation of the air-fuel ratio control device may occur if the finely polished surface of the precision valve therein contains burrs due to faulty manufacture or if the precision valve encounters dirt or other abrasive debris in the circulated engine lubrication oil. Third, it is undesirable that the above air-fuel ratio control device depends upon air intake manifold pressure for activation following engine startup since the time lag required for attaining a predetermined air intake manifold pressure unduly prolongs the period in which a smokey engine exhaust is produced.
The present invention is directed to overcoming one or more of the problems as set forth above.